In an engine control such as a diesel engine (hereinafter, engine) used for a work machine, when an operator of the work machine sets a fuel adjustment dial (throttle dial) provided in a cab to any position, the engine controller outputs, to a fuel injection system, a control signal for injecting the amount of fuel injection in accordance with setting to the engine. The engine controller outputs a control signal in accordance with change of the load of the working machine attached to the work machine and adjusts the engine speed, so as to maintain the target engine speed set by the fuel adjustment dial (throttle dial). The engine controller or a pump controller calculates a target absorption torque of the hydraulic pump in accordance with the target engine speed. This target absorption torque is set such that the output horsepower of the engine matches the absorption horsepower of the hydraulic pump.
Ordinary engine control will be explained with reference to FIG. 26. The engine is controlled so as not to exceed an engine output torque line TL constituted by a maximum output torque line P1 of the engine and an engine droop line Fe subtracted from the maximum engine speed. For example, when the work machine is an excavator, the engine controller generates a control signal for changing the engine speed in accordance with rotation operation of the upper rotation body, the amount of manipulation of a manipulation lever manipulated for working machine operation, the load of the working machine, and the like. For example, when excavation operation of soil and the like is performed while the target engine speed is as N2, an engine speed (idling engine speed N1) while the engine is in idling operation is changed to a target engine speed N2. At this occasion, in response to the control signal from the engine controller, the fuel injection system injects fuel to the engine in accordance with this change, and when working machine operation and the like is performed to increase the load, the engine speed is changed so that the engine speed and the engine output torque reaches a matching point M1 corresponding to a crossing point of an engine output torque line TL and a pump absorption torque line PL of a variable displacement hydraulic pump (typically, swash plate hydraulic pump). At a rated point P, the engine output is the maximum.
In this case, in order to improve fuel-efficiency of the engine and the pump efficiency of the hydraulic pump, there is an engine control device as illustrated in FIG. 27, in which a target engine driving line (target matching route) ML passing a region where a fuel consumption rate is high is made, and a matching point of the engine output and the pump absorption torque is taken on this target matching route ML. In FIG. 27, a curved line M denotes an equal fuel-efficiency curved line of an engine and the like, and a point closer to the center of the curved line M (eye (M1)) is more advantageous in the fuel consumption rate. The curved line J denotes an equal horsepower curved line at which the horse power absorbed by the hydraulic pump is the equal horsepower. Therefore, in order to obtain a same horsepower, the fuel consumption rate is better if matching is obtained at the matching point pt2 on the target matching route ML as compared with the matching point pt1 on the engine droop line Fe. A flow rate Q of the hydraulic pump is a product of the engine speed n and a pump capacity q (Q=n·q), and in order to obtain a same operating oil flow rate, the pump efficiency is better when the engine speed is reduced and the pump capacity is increased.